1. Field of the Invention
This invention relates to a method for formation of a functional film, particularly a semiconductive deposited film which is can be used in a semiconductor device, photosensitive device for electrophotography, electronic device such as optical input sensor device for optical image inputting device, etc.
2. Related Background Art
Hitherto, for functional films, especially amorphous or polycrystalline semiconductive films, individually suitable film forming methods have been employed from the standpoint of desired physical characteristics, uses, etc.
For example, for formation of silicon type deposited films such as amorphous or polycrystalline non-single crystalline silicon which are optionally compensated for lone pair electrons with a compensating agent such as hydrogen atoms (H) or halogen atoms (X), etc., (hereinafter abbreviated as "NON-Si (H,X)", particularly "A-Si (H,X)" when indicating an amorphous silicon and "poly-Si (H,X)" when indicating a polycrystalline silicon) (the so-called microcrystalline silicon is included within the category of A-Si (H,X) as a matter of course), there have been attempted the vacuum vapor deposition method, the plasma CVD method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc. Generally, the plasma CVD method has been widely used and industrialized.
However, the reaction process in formation of a silicon deposited film according to the plasma CVD method which has been generalized up to now is considerably complicated as compared with the conventional CVD method, and its reaction mechanism is not well understood. Also, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rate and flow rate ratio of the introduced gases, pressure during formation, high frequency power, electrode structure, structure of the reaction vessel, speed of evacuation, plasma generating system, etc.). Because of the combination of such a large number of parameters, the plasma may sometimes become unstable state, whereby marked deleterious influences were frequently exerted on the deposited film formed. Besides, the characteristic parameters must be selected for each device and therefore under the present situation it has been difficult to generalize the production conditions.
On the other hand, for the silicon type deposited film to exhibit sufficiently satisfactory electric and optical characteristics for respective uses, it is now accepted that the best to form it according to the plasma CVD method.
However, depending on the application use of the silicon type deposited film, bulk production with reproducibility must be attempted with full satisfaction of enlargement of area, uniformity of film thickness as well as uniformity of film quality, and therefore in formation of a silicon type deposited film according to the plasma CVD method, enormous installation investment is required for a bulk production device and also management for such bulk production becomes complicated, with management tolerance being narrow, and the control of the device being complex. These are pointed out as the problems to be improved in the future.
Also, in the case of the plasma CVD method, since plasma is directly generated by high frequency or microwave, etc., in the film forming space in which a substrate on which film is formed is arranged, electrons or a number of ion species generated may give damages to the film in the film forming process resulting in lowering of film quality or non-uniformization of film quality.
As an improvement of this point, the indirect plasma CVD method has been proposed.
The indirect plasma CVD method has elaborated to use the principal substance for formation of deposited film by forming an activated species of the principal substance for formation of deposited film by microwave, etc., at an upstream position apart from the film forming space and transporting said activated species to the film forming space.
However, even by such a plasma CVD method, transport of activated species is essentially required and therefore the activated species effective for film formation must have long life, whereby kinds of gases which can be employed are substantially limited, thus failing to provide various deposited films. Also, enormous energy is required for generation of plasma, and generation of the chemical species effective for film formation and their amounts cannot be essentially placed under simple management. Thus, vairous problems remain to be solved.
As contrasted to the plasma CVD method, the optical CVD method is advantageous in that no ion species or electrons are generated which cause damage to the film quality during film formation. However, there are problems when the light source does not include wave lengths tending toward the UV range, that a large scale light source and its power source are required in the case of industrialization, that the window for permitting the light from the light source to be introduced into the film forming space is coated with a film during film formation to result in lowering in dose during film formation, which may further lead to shut-down of the light from the light source into the film forming space.
As described above, in formation of silicon type deposited film, the points to be solved still remain, and it has been earnestly desired to develop a method for forming a deposited film which is capable of bulk production by attempting to save energy by means of a device of low cost, while maintaining the characteristics as well as uniformity which are practicably available. Especially, the above points are highly demanded when forming a semiconductor film of the p, n or i conduction type while enhancing the doping rate therein. These are also applicable in the case of enhancing the doping rate in other functional films, for example, semiconductive films such as silicon type films including silicon nitride films, silicon carbide films, silicon oxide films or germanium type films as the similar problems which should be solved respectively.